Increased Nitrogen Fixation Research Articles

Impact of Climate Change on Ecological Quality Indicators and Biogeochemical Fluxes in the Baltic Sea: A Multi-Model Ensemble Study

Multi-model ensemble simulations using three coupled physical-biogeochemical models were performed to calculate the combined impact of projected future climate change and plausible nutrient load changes on biogeochemical cycles in the Baltic Sea. Climate projections for 1961-2099 were combined with four nutrient load scenarios ranging from a pessimistic business-as-usual to a more optimistic case following the Helsinki Commission's (HELCOM) Baltic Sea Action Plan (BSAP). The model results suggest that in a future climate, water quality, characterized by ecological quality indicators like winter nutrient, summer bottom oxygen, and annual mean phytoplankton concentrations as well as annual mean Secchi depth (water transparency), will be deteriorated compared to present conditions. In case of nutrient load reductions required by the BSAP, water quality is only slightly improved. Based on the analysis of biogeochemical fluxes, we find that in warmer and more anoxic waters, internal feedbacks could be reinforced. Increased phosphorus fluxes out of the sediments, reduced denitrification efficiency and increased nitrogen fixation may partly counteract nutrient load abatement strategies.

No stimulation of nitrogen fixation by non-filamentous diazotrophs under elevated CO2 in the South Pacific.

Nitrogen fixation by diazotrophic cyanobacteria is a critical source of new nitrogen to the oligotrophic surface ocean. Research to date indicates that some diazotroph groups may increase nitrogen fixation under elevated pCO2 . To test this in natural plankton communities, four manipulation experiments were carried out during two voyages in the South Pacific (30-35o S). High CO2 treatments, produced using 750ppmv CO2 to adjust pH to 0.2 below ambient, and 'Greenhouse' treatments (0.2 below ambient pH and ambient temperature +3°C), were compared with Controls in trace metal clean deckboard incubations in triplicate. No significant change was observed in nitrogen fixation in either the High CO2 or Greenhouse treatments over 5day incubations. qPCR measurements and optical microscopy determined that the diazotroph community was dominated by Group A unicellular cyanobacteria (UCYN-A), which may account for the difference in response of nitrogen fixation under elevated CO2 to that reported previously for Trichodesmium. This may reflect physiological differences, in that the greater cell surface area:volume of UCYN-A and its lack of metabolic pathways involved in carbon fixation may confer no benefit under elevated CO2 . However, multiple environmental controls may also be a factor, with the low dissolved iron concentrations in oligotrophic surface waters limiting the response to elevated CO2 . If nitrogen fixation by UCYN-A is not stimulated by elevated pCO2 , then future increases in CO2 and warming may alter the regional distribution and dominance of different diazotroph groups, with implications for dissolved iron availability and new nitrogen supply in oligotrophic regions.

Improvement of peanut rhizobial inoculant by incorporation of plant growth promoting rhizobacteria (PGPR) as biocontrol against the seed borne fungus, Aspergillus niger

The inhibition of Aspergillus niger that causes root rot diseases in peanut (Arachis hypogaea L.) was investigated by using 765 bradyrhizobial and 350 soil-isolated plant growth promoting rhizobacteria (PGPR) strains as biological controllers. Only 11 PGPR isolates were found to be able to inhibit A. niger growth. Based on their ability to inhibit A. niger growth and root colonization, the best four PGPR isolates, A20, A45, A62, and A106, were selected, and their 16S rRNA sequences were highly homologous to Bacillus megaterium, B. subtilis, B. subtilis subsp. subtilis, and Pseudomonas sp., respectively. The production of a lytic protease enzyme was detected in A20, A45, and A62, but not in A106. Some antifungal activities were clearly found in cell-free supernatants of A20 and A62. Interestingly, the antifungal activity of isolates A45 and A62 was proteinase K resistant. All PGPR isolates could produce an auxin (indole-3-acetic acid, IAA) hormone and biofilms. IAA produced from PGPR isolates could clearly promote peanut root growth. When either isolate A20 or A45 (108cellsper ml) was co-inoculated with Bradyrhizobium sp. TAL 173 (108cells per ml), the peanut root rot disease caused by A. niger (105 and 106spores per seed) could be inhibited. Incorporating rhizobia with selected PGPR increases nitrogen fixation and reduces fungicide usage in peanut, providing an appropriate approach for sustainable agriculture.

Cyanolichens can have both cyanobacteria and green algae in a common layer as major contributors to photosynthesis.

Cyanolichens are usually stated to be bipartite (mycobiont plus cyanobacterial photobiont). Analyses revealed green algal carbohydrates in supposedly cyanobacterial lichens (in the genera Pseudocyphellaria, Sticta and Peltigera). Investigations were carried out to determine if both cyanobacteria and green algae were present in these lichens and, if so, what were their roles. The types of photobiont present were determined by light and fluorescence microscopy. Small carbohydrates were analysed to detect the presence of green algal metabolites. Thalli were treated with selected strengths of Zn(2+) solutions that stop cyanobacterial but not green algal photosynthesis. CO(2) exchange was measured before and after treatment to determine the contribution of each photobiont to total thallus photosynthesis. Heterocyst frequencies were determined to clarify whether the cyanobacteria were modified for increased nitrogen fixation (high heterocyst frequencies) or were normal, vegetative cells. Several cyanobacterial lichens had green algae present in the photosynthetic layer of the thallus. The presence of the green algal transfer carbohydrate (ribitol) and the incomplete inhibition of thallus photosynthesis upon treatment with Zn(2+) solutions showed that both photobionts contributed to the photosynthesis of the lichen thallus. Low heterocyst frequencies showed that, despite the presence of adjacent green algae, the cyanobacteria were not altered to increase nitrogen fixation. These cyanobacterial lichens are a tripartite lichen symbiont combination in which the mycobiont has two primarily photosynthetic photobionts, 'co-primary photobionts', a cyanobacterium (dominant) and a green alga. This demonstrates high flexibility in photobiont choice by the mycobiont in the Peltigerales. Overall thallus appearance does not change whether one or two photobionts are present in the cyanobacterial thallus. This suggests that, if there is a photobiont effect on thallus structure, it is not specific to one or the other photobiont.

Moss-specific changes in nitrogen fixation following two decades of warming, shading, and fertilizer addition

Climate warming will induce changes in Arctic ecosystem carbon balance, but besides climate, nitrogen availability is a critical controlling factor of carbon cycling. It is therefore essential to obtain knowledge on the influence of a changing climate on nitrogen fixation, as this process is the main source of new nitrogen to arctic ecosystems. In order to gain information on future nitrogen fixation rates in a changing climate, we studied the effects of two decades of warming with passive greenhouses, shading with sackcloth, and fertilization with NPK fertilizer on nitrogen fixation rates. To expand the knowledge on species-specific responses, we measured nitrogen fixation associated with two moss species: Hylocomium splendens and Aulacomnium turgidum. Our expectations of decreased nitrogen fixation rates in the fertilizer and shading treatments were met. However, contrary to our expectation of increased nitrogen fixation in the warming treatment, we observed either no change (Hylocomium) or a decrease (Aulacomnium) in fixation in the warmed plots. We hypothesize that this could be due to moss-specific responses or to long-term induced effects of the warming. For example, we observed that the soil temperature increase induced by the warming treatment was low and insignificant as vegetation height and total vascular plant cover of the warmed plots increased, and moss cover decreased. Hence, truly long-term studies lasting more than two decades provide insights on changes in key biogeochemical processes, which differ from more transient responses to warming in the Arctic.

Elevated foraminifera‐bound nitrogen isotopic composition during the last ice age in the South China Sea and its global and regional implications

We report a new foraminifera‐boundδ15N (FB‐δ15N) record from the South China Sea (SCS) extending back to 42 ka. This record shows a ∼1.2‰ glacial‐to‐interglacialδ15N decrease, with a deglacial δ15N maximum similar to that observed in many bulk sedimentary δ15N records and in a Caribbean FB‐δ15N record. The glacial‐to‐interglacialδ15N decrease is smaller than in the Caribbean record, indicating that at least half of the Caribbean δ15N decrease into the Holocene was regional, not global, supporting the interpretation of a Holocene increase in Atlantic nitrogen fixation. At the same time, the glacial‐to‐interglacialδ15N decrease observed in the SCS may also be explained as a regional signal of increasing nitrogen fixation into the Holocene. Other aspects of the SCS record suggest an effect of physical circulation on FB‐δ15N. FB‐δ15N starts to increase toward its deglacial maximum before the last ice age ends, and it continues to decrease through the later Holocene, in contrast to the more pure glacial/deglacial/interglacial steps in the Caribbean record. These changes have parallels in other Pacific δ15N records, precessionally driven East Asian monsoon records, and the slow Holocene warming of SCS surface waters. In addition, the SCS record exhibits two one‐point high‐δ15N spikes in multiple foraminiferal species that coincide with apparent hydrographic events. Thinning or weakening of the thermocline, in the late glacial and early Holocene as well as during the noted events, may have yielded a higher δ15N for thermocline nitrate and thus a higher FB‐δ15N.

Effects of Water Table Control by Farm-Oriented Enhancing Aquatic System on Photosynthesis, Nodule Nitrogen Fixation, and Yield of Soybeans

We evaluated the effects of the water table controlled by a water table controlling system, farm-oriented enhancing aquatic system (FOEAS), on soybean productivity. FOEAS was constructed in a heavy soil paddy field; we made plots with watertables maintained at −32 (water table depth [WTD]32) and −20 cm (WTD20) from the soil surface and, as a control, an open-ditch drained plot (ODD). Soybean cultivar “Tachinagaha” and non-nodulating cultivar “En1282” were cultivated in 2006 and 2007. The moisture of the topsoil in the water table-controlled plots showed lessfluctuation owing to rainfall. The roots and nodules were distributed only in the upper soil layer in ODD, but more weredistributed in deeper soil layers in WTD32 and WTD20. In Tachinagaha, the SPAD value and stomatal conductance were higher inWTD32 and WTD20 than in ODD, resulting in a higher apparent photosynthetic rate. The aboveground total dry weight and nitrogen accumulation of Tachinagaha were higher in the water table-controlled plots than in ODD; in En1282, this difference was insignificant. The relative ureide value which is an index of nodule activity, in ODD was depressed by both excess and deficit soil moisture; however, that in WTD32 remained relatively high during the growing stage. Tachinagaha showed higheryield in FOEAS plots, especially in WTD32 than in ODD in both years. The results indicate that control of water table by FOEAS increases nitrogen fixation, photosynthesis, and yield of soybeans in upland fields converted from paddy fields.

Biogeochemical response of alpine lakes to a recent increase in dust deposition in the Southwestern, US

Abstract. The deposition of dust has recently increased significantly over some regions of the western US. Here we explore how changes in dust deposition have affected the biogeochemistry of two alpine watersheds in Colorado, US. We first reconstruct recent changes in the mass accumulation rate of sediments and then we use isotopic measurements in conjunction with a Bayesian mixing model to infer that approximately 95% of the inorganic fraction of lake sediments is derived from dust. Elemental analyses of modern dust indicate that dust is enriched in Ca, Cr, Cu, Mg, Ni, and in one watershed, Fe and P relative to bedrock. The increase in dust deposition combined with its enrichment in certain elements has altered the biogeochemisty of these systems. Both lakes showed an increase in primary productivity as evidenced by a decrease in carbon isotopic discrimination; however, the cause of increased primary productivity varies due to differences in watershed characteristic. The lake in the larger watershed experienced greater atmospheric N loading and less P loading from the bedrock leading to a greater N:P flux ratio. In contrast, the lake in the smaller watershed experienced less atmospheric N loading and greater P loading from the bedrock, leading to a reduced N:P flux ratio. As a result, primary productivity was more constrained by N availability in the smaller watershed. N-limited primary productivity in the smaller watershed was partly ameliorated by an increase in nitrogen fixation as indicated by reduced nitrogen isotopic values in more contemporary sediments. This study illustrates that alpine watersheds are excellent integrators of changes in atmospheric deposition, but that the biogeochemical response of these watersheds may be mediated by their physical (i.e. watershed area) and chemical (i.e. underlying geology) properties.

Response of surface nutrient inventories and nitrogen fixation to a tropical cyclone in the southwest Pacific

To examine the controls of new nitrogen supply in oligotrophic waters, nitrogen fixation and nutrients were monitored during a quasi‐Lagrangian study in surface waters of the north Tasman Sea during March–April 2006. The study was initiated coincident with a tropical cyclone, with high rainfall and winds that eroded surface layer density structure and increased surface phosphate and silicate concentrations by 50%. However, there was no corresponding increase in nitrate, as the enhanced vertical mixing perturbed the phosphacline but not the deeper nitracline. Dissolved iron was highest in the upper 20 m, and declined to a minimum at 100–150 m, indicating wet deposition as the source, as also inferred from the salinity decrease. Nitrogen fixation increased significantly by an order of magnitude over the following 9 d. In deckboard perturbation experiments immediately after the cyclone, phosphate and iron addition did not elicit a response in nitrogen fixation, whereas 8 d later, when in situ dissolved iron had declined, the addition of iron stimulated nitrogen fixation by 1.5‐fold. Conversely, the addition of dust from Australia and the Gobi desert immediately after the cyclone increased nitrogen fixation by 2‐ and 4.5‐fold, respectively. The observed 10‐fold increase in nitrogen fixation in situ exceeded that reported in other nutrient perturbation experiments despite comparatively low in situ dissolved iron, indicating that iron bioavailability may be a critical factor. Although stimulation by a cyclone was unexpected, it promoted favorable conditions for diazotrophy by enhancing phosphate availability in the absence of nitrate, and increased dissolved iron supply and availability via wet deposition. This suggests that future increases in tropical cyclones, combined with other climate‐driven trends, may increase nitrogen fixation in the north Tasman Sea.

Symbiogenetics and breeding of a macrosymbiont for increased nitrogen fixation capacity with special reference to the pea (Pisum sativum L.)

This article presents the results of long-term experimental studies on symbiogenetics and breeding of macrosymbionts in a system of legume-rhizobial symbiosis with the pea (Pisum sativum L.) as an example. The collection of pea symbiotic mutants was created and genetically studied. The new symbiotic genes were identified and localized on a Pisum chromosome map. The dominant symbiotic mutants characterized by high productivity and active nitrogen fixation were induced in the pea for the first time. Certain physiological peculiarities of the auxin and gibberellin phytohormone content were studied in symbiotic mutants. The differences in the activity of nitrogen fixation during ontogeny were revealed for numerous mutants. The forms with a prolonged period of active nitrogen fixation were isolated. A new approach to estimate the efficiency of legume-rhizobial symbiosis was proposed. The method of use of symbiotic mutants in breeding for higher efficiency of nitrogen fixation was proposed for the first time. This method is based on the interaction between different sym genes within one genotype. A series of productive constant recurrent lines of the seventh generation was designed which exceed commercial pea cultivars in nodulation and nitrogen fixation activity. Lines are characterized by accumulation of higher root biomass with increased nitrogen content in them. This approach allows their use in agricultural production instead of expensive organic and mineral fertilizers.

Mutational search for high temperature (60<sup>o</sup>C) tolerant variant of Rhizobium species CWP G34A—Mutation generates high temperature variant of Rhizobium species Cwp G34A

This study focused on the development of thermophilic strain/s of a cowpea (Vigna unguiculata) compatible nitrogen fixing bacterium. A preliminary plant screening was carried out using some strains of tropical rhizobia and cowpea. Rhizobium species CWP G34A that formed Fix+ nodules repeatedly was selected for further studies. First, it was tested for growth at high temperatures of 40 to 55oC at 5oC interval with 28oC as the control temperature. Mutagenesis was conducted on the bacterium with ethylmethane sulphonate (EMS). The wildtype and mutants generated were tested for high temperature tolerance by growing them individually in nutrient broth at 60oC for 24 hours. Optical density (670 nm) was read before and after incubation. The mutants were grouped into classes based on percentage difference in OD values obtained before and after exposure to 60oC. Rhizobium species CWP G34A produced functional pink nodules on the cowpea consistently in three different plant tests. There was no growth at all the temperatures tested except at 28oC and 40oC after 24 hours of incubation. It grew better at former (51 × 1010 Cfu/ml) than latter (11 Cfu/ml) temperature. Like the parental strain, all the mutants but one, did not grow after exposure to 60oC. Sixty degree centigrade caused various reductions in optical density (OD) values of the variants. Eleven classes of the mutants were formed with membership percentage ranging from 1 to 22%. Class 1 contains only one member while class 11 has the highest mutant population of 22% with OD difference of 0 to 10% and –90 to –100% respectively. The high percentage reduction in the OD of variants in class 11 is similar to that of the unmutated cells (–94.56%). The only mutant that survived the 60oC and grew was MU70. An increase of 1.67% in OD was obtained for MU70. Mutant MU70 therefore appeared a promising strain that can be further tested to inoculate cowpea in the dry and warm season for increased nitrogen fixation. This will provide encouraging information for farmers to grow the cowpea throughout the year particularly under high temperatures in summer in order to boost the yield of the legume.

Cultivar effects on nitrogen fixation in peas and lentils

Increasing nitrogen fixation in legume crops could increase cropping productivity and reduce nitrogen fertilizer use. Studies have found that crop genotype, rhizobial strain, and occasionally genotype-specific interactions affect N fixation, but this knowledge has not yet been used to evaluate or breed for greater N fixation in US crops. In this study five USDA varieties of lentils (Lens culinaris Medik.) and five varieties of peas (Pisum sativum L.) were tested with 13 to 15 commercially available strains of Rhizobium leguminoserum bv. viciae to identify the better N fixing rhizobial strains, crop varieties, and specific pairings. Peas and lentils inoculated with individual strains were grown in growth chambers for 6 week. Plants received (15NH4)2 SO4 (5 at.%) starter fertilizer to measure N fixation by isotope dilution. Below- and above-ground biomass, numbers of nodules, and the proportion of plant N supplied by fixation (PNF) were determined. The percent of N fixed was significantly affected by crop variety and significantly correlated with number of nodules in both lentils and peas. This implies that one strategy for enhancing crop N fixation is developing varieties that have higher rhizobium infection rates. Total N fixation in lentils was significantly influenced by both crop variety and rhizobial strain. Eston variety lentil and Shawnee variety pea had the highest PNF of 80.8% and 91.3%, respectively. The different strains of R. leguminoserum affected PNF in lentils but not in peas. These findings suggest that N fixation improvement in lentils and peas may be addressed most effectively by breeding crops for greater N fixation hosting capacity.

Interspecific competition enhances nitrogen fixation in an actinorhizal shrub

In forest understory restoration, the establishment of reintroduced species may be strongly linked to their ability to compete for belowground resources. In this study, we provide isotopic and morphological evidence for competition-induced increases in nitrogen fixation by Morella cerifera (L.) Small (wax myrtle) when planted with Pinus palustris Mill (longleaf pine). Compared to a competition-free treatment, we found no significant differences in tissue N concentrations for M. cerifera. However, 15N enrichment in leaves, stems and roots, as well as whole-plant values for nitrogen derived from fertilizer were significantly lower when the plants were subject to interspecific competition from P. palustris. Plants in the competition treatment also allocated a significantly greater percentage of belowground biomass to root nodules than those in the competition-free treatment (0.65 vs. 0.41%). This strongly suggests that M. cerifera is capable of upregulating nitrogen fixation in response to interspecific competition. This may help explain why M. cerifera outperformed non-nitrogen-fixing species reintroduced on the same site.

Nitrogen Isotopic Records of Terrestrial Pollution Encoded in Floridian and Bahamian Gorgonian Corals

Stable nitrogen isotope (delta(15)N) analysis has proven an effective "fingerprint" of sewage contamination in coral reef environments; however, short-term variability in nitrogen cycling and isotopic fractionation may obscure long-term trends. Here, we examine delta(15)N signatures in the organic endoskeletons of long-lived (20-40 years) gorgonian corals. Specimens were collected from relatively pristine reefs off Green Turtle Cay, Bahamas, and from reefs off southeast Florida heavily impacted by multiple sources of anthropogenic nitrogen. The delta(15)N of the most recently grown skeleton (branch tips) ranged from +2 to +3 per thousand at Green Turtle Cay, and +4.5 to +10 per thousand off Florida. These values closely match the delta(15)N of macroalgae collected from the same locations, indicating that gorgonian corals are isotopically similar to primary producers, and therefore suitable for assessing sources of dissolved inorganic nitrogen. Differences in the delta(15)N between younger and older skeleton indicated an overall decline of -0.34 +/- 0.06 per thousand (1 s.e) over the last 20 - 40 years at Green Turtle Cay, reflecting a possible increase in nitrogen fixation and/or atmospheric deposition of anthropogenic nitrogen. Off southeast Florida, there was an overall increase in delta(15)N over the same time period, reflecting increasing wastewater discharges from the rapidly growing population. These results highlight the usefulness of delta(15)N recorded in gorgonians and other long-lived organisms in assessing spatiotemporal patterns of nitrogen sources to coastal marine environments.

Herbivory by an invasive snail increases nitrogen fixation in a nitrogen-limited stream

Despite anthropogenic nitrogen contributions, nitrogen fixation contributes half of biosphere inputs but has rarely been quantified in streams. Herbivory controls algal biomass and productivity in streams, and we hypothesized that herbivory could also control nitrogen fixation. We released periphyton from herbivory in nitrogen-limited Polecat Creek, Wyoming, where heavy grazing by the invasive New Zealand mudsnail ( Potamopyrgus antipodarum ) dominates nitrogen cycling. One and two weeks after releasing periphyton, we found higher rates of nitrogen fixation on heavily grazed rocks (two-way analysis of variance (ANOVA), p = 0.012). Time elapsed after snail manipulation had no effect (two-way ANOVA, p = 0.24). Grazing changed periphyton composition by reducing the proportion of green algae and increasing the proportion of nitrogen-fixing diatoms (multivariate ANOVA, p = 0.001). Nitrogen fixation rates increased disproportionately to nitrogen-fixing algal cells, indicating that snails increased nitrogenase efficiency, probably by improving light and (or) nutrient availability to nitrogen fixers. We incorporated our nitrogen fixation rates into a published nitrogen budget for Polecat Creek and found that nitrogen flux into the periphyton was 50% higher when we included nitrogen fixation. Herbivory can increase nitrogen fixation in streams, and future studies should measure nitrogen fixation for a more thorough understanding of stream nitrogen cycling.

The unbearable naivety of legumes in symbiosis

The mechanisms by which legumes choose their rhizobial partners operate independently from their ability to fix nitrogen. As a result of this naivety, symbiotic nitrogen fixation is often suboptimal. The initial recognition of the bacterial partner and the subsequent signal transduction in the host root utilises components that are functionally conserved between legumes and probably actinorhiza host plants. However, the later steps, which largely determine symbiotic performance, are subject to ongoing evolutionary diversification of molecular mechanisms. For example, the impact of bacterial effector proteins, the occurrence of terminal bacteroid differentiation and the expression of bacterial hydrogenase, all depend on the plant genotype. Strategies towards increased nitrogen fixation of legumes in agriculture need to encompass this diversification of mechanisms.

Variation in moss-associated nitrogen fixation in boreal forest stands

Traditionally it has been thought that most boreal forest communities lack a significant input of biologically fixed nitrogen. Recent discoveries of nitrogen fixation by cyanobacteria associated with mosses have resulted in a re-evaluation of this view. While it is recognized that rates of nitrogen fixation in mosses can be highly variable, there is little understanding as to why this occurs. I monitored nitrogen fixation, using acetylene reduction, in wet lowland and dry upland boreal forest communities, in central Canada, over a growing season. At the peak of nitrogen fixation in mid summer, Sphagnum capillifolium had an 11 times higher rate of fixation than Pleurozium schreberi. Variation in canopy openness and precipitation had no effect on rates of fixation over the growing season. In P. schreberi fixation rates did not vary between sites. Temperature had a positive effect on fixation rates in both S. capillifolium and P. schreberi, but the effect was 4 times more pronounced in S. capillifolium. Seasonal rates of nitrogen fixation were estimated at 193 mg N m(-2) for S. capillifolium and 23 mg N m(-2) for P. schreberi. With moderate increases in climate warming, predicted increases in nitrogen fixation in S. capillifolium are sufficient to raise its decomposition rate. Increased temperatures may therefore act synergistically to change boreal systems from a sink to a source of carbon.

Nitrogen sources in the South China Sea, as discerned from stable nitrogen isotopic ratios in rivers, sinking particles, and sediments

Stable nitrogen isotopic ratios were measured in sinking particles and surface sediments from the South China Sea (SCS) in order to study recent nitrogen sources and degradation. Average δ 15N values of 16 sediment traps deployed at seven locations in the northern, central and southern SCS were uniformly low, ranging between 2.7 and 4.5‰ with a winter minimum in the northern and central SCS. Enhanced nitrogen contents and δ 15N values were noted in samples affected by swimmers, comprising between 5 and 20% of total nitrogen fluxes. Nitrate sources were subsurface waters from the western Pacific, which were isotopically depleted due to the remineralization of nitrogen from nitrogen fixation in surface waters. Nitrogen fixation in the SCS contributed up to 20% to the settling particles. In the southern SCS, resuspended matter close to the shelf added to the sinking particulates. The long-term trap record from the central SCS revealed decreasing δ 15N values during the 1990s, which correspond with findings from the North Pacific Subtropical Gyre and may be attributable to increased nitrogen fixation due to global warming-related stratification. This trend may be restricted to the 1990s but could also persist due to the projection of more frequent occurrence of El Niño conditions. The δ 15N increase from swimmer-free trap averages of 2.7–3.6‰ to values of 5–6‰ in underlying deep-sea sediments was in the same range as in other deep ocean areas. Similar to results from the northern Indian Ocean, this increase could be related to isotopic enrichment during amino acid degradation. The lowest sedimentary δ 15N values characterize the Pinatubo ash layer deposited off Luzon in an event of mass sedimentation in 1991. The fast deposition of organic matter drawn from the surface waters with the ash in the form of vertical density currents evidently preserved the planktonic δ 15N signal.

Farklı Bakteri (Bradyrhizobium japonicum) Aşılama Yöntemlerinin Soyada Azot Fiksasyonu ve Tane Verimine

Bu araştırma, farklı bakteri aşılama yöntemlerinin soyada azot fiksasyonu ve tane verimlerine etkilerini belirlemek amacıyla deneme iklimlendirme kabininde yürütülmüştür. Bakteriyel aşılamalar basit tohuma aşılama, üst aşılama, 2 defa üst aşılama, tohum yatağına pulverize aşılama ve peat ile aşılama olarak uygulanmıştır. Çalışma tesadüf parselleri deneme desenine göre 8 tekrarlamalı saksı denemesi şeklinde yürütülmüştür. Deneme materyali olarak Sa88 soya çeşidi tohumları ile Bradyrhizobium japonicum bakterisi 1809 nolu suşu kullanılmıştır. Çiçeklenme ve hasat olgunluğu döneminde nodülasyon gözlemleri ve bitki analizleri yapılmıştır. Denemeden elde edilen sonuçlar incelendiğinde; soyada uygulanan bakteri aşılama yöntemlerinin azot fiksasyonunu arttırdığı ve bu yöntemlerin pratiğe aktarılabilir olduğu belirlenmiştir. En yüksek bitki tane sayısı 9,08 adet/bitki ve bitki tane verimi 3,36 g/bitki tohum yatağına aşılama uygulamasından elde edilmiştir

Eutrophication in the Baltic Sea and shifts in nitrogen fixation analyzed with a 3D ecosystem model

Since the middle of the last century the Baltic Sea ecosystem has undergone a strong change. An obvious indicator is the increase of winter nutrient concentrations. This increase is attributed to increased anthropogenic nutrient loads to the Baltic Sea. With a 3D ecosystem model we made a hindcast simulation of eutrophication from 1960 to 2000. The model system was able to reproduce the main hydrographic and ecologic features of this period. However, the observed strong increase in winter nutrient concentrations was underestimated by the model. The simulated nitrogen fixation shows a pronounced interdecadal variability. Nitrogen fixation increased in the early 1990 at the same time nutrient loads to the Baltic Sea were decreasing. The changes in nutrient loads cannot fully explain the increased nitrogen fixation; in fact, the primary trigger for this increase is an intensified wind speed in winter, which is correlated with changes in the North Atlantic oscillation (NAO).